The amount of power that can be delivered by certain alternative energy sources, such as photovoltaic cells (“PV cells” or “solar cells”), may vary in magnitude over time owing to temporal variations in operating conditions. For example, the output of a typical PV cell will vary with variations in sunlight intensity, angle of incidence of sunlight, ambient temperature and other factors. One application of alternative energy sources is delivery of power to an alternating-current (AC) utility grid. In such applications, an inverter (i.e., a DC-AC power conditioner) is required in order to turn the DC power delivered by the alternative energy source into sinusoidal alternating-current (AC) power at the grid frequency. Certain inverters (e.g., those used by residential customers or small businesses) convert the DC power delivered by the alternative energy source into single-phase AC power and deliver a sinusoidal current to the AC grid at the grid frequency. One figure of merit for such an inverter is the utilization ratio, which is the percentage of available power that it can extract from an energy source. Ideally, an inverter will achieve a utilization ratio of 100%.
Some photovoltaic power systems comprise strings of solar cells that deliver relatively high DC voltages (e.g., nominal 450V). Because operating characteristics of cells in a large string will typically differ, and because individual cells may receive different amounts of sunlight, it is difficult or impossible to run large strings at the combined full power capacity of the individual cells. Strings also typically produce significant power (e.g., kilowatts), which requires a large inverter and high voltage wiring between the cells and the inverter. Additionally, failure of one inverter results in loss of power from the entire string.
Distributed photovoltaic power systems seek to overcome the problems of older, string-based, systems. In a distributed system, each one of a plurality of small, relatively low voltage (e.g. 25V), solar panels are provided with an inverter that feeds power to the AC grid. The benefits of such systems include improved utilization of total available solar cell power and a high level of redundancy: failure of a single inverter or panel may not result in a significant loss in delivered power.
Conversion efficiency in a distributed photovoltaic power system may be improved by placing the inverter in close proximity to its solar panel, thereby eliminating the need to bus a plurality of relatively high solar panel currents to remotely located inverters for processing. Putting the inverter next to its solar panel, however, exposes the inverter to relatively high operating temperatures and may make it more difficult to access for maintenance.
A basic electrical property of a single-phase AC power system is that the energy flow includes both an average power portion that delivers useful energy from the energy source to the load and a double-frequency portion that flows back and forth between the load and the source:p(t)=Po+Po*cos(2ωt+Φ)  (1)
In applications involving inverters, the double-frequency portion represents undesirable ripple power that, if reflected back into the DC power source, may compromise performance of the source. This is particularly true for photovoltaic cells.
Photovoltaic cells have a single operating point at which the values of the current and voltage of the cell result in a maximum power output. This “maximum power point” (“MPP”) is a function of environmental variables, including light intensity and temperature. Inverters for photovoltaic systems typically comprise some form of maximum power point tracking (“MPPT”) as a means of finding and tracking the maximum power point (“MPP”) and adjusting the inverter to exploit the full power capacity of the cell at the MPP. Extracting maximum power from a photovoltaic cell requires that the cell operate continuously at its MPP; fluctuations in power demand, caused, for example, by double-frequency ripple power being reflected back into the cell, will compromise the ability of the inverter to deliver the cell's maximum power. One analysis has shown that that the amplitude of the ripple voltage across a photovoltaic module should be below 8.5% of the MPP voltage in order to reach a utilization ratio of 98%. Thus, it is preferable that inverters for photovoltaic energy systems draw only the average power portion of the energy flow from the photovoltaic cells at the inverter input. Such inverters should therefore comprise means to manage the double-frequency ripple power without reflecting it back into the source.
To manage double-frequency ripple power, energy needs to be stored and delivered at twice the AC frequency. One way to manage the double-frequency ripple power is to use passive filtering in the form of capacitance across a DC bus. This passive filtering arrangement requires a large capacitance value to filter the double-frequency power, since the energy exchange needs to be supported without imposing significant voltage ripple on the DC bus.
Another way to manage double-frequency ripple power is to use an active filter circuit that supplies the double-frequency ripple power by means of a capacitor internal to the active filter. Whereas the passive filtering approach requires a relatively large filter capacitor, the internal capacitor in an active filter may be made relatively much smaller, since it is only required to store and deliver the double-frequency ripple power and is not required to support the DC bus voltage. Because the active filter “isolates” the internal capacitor from the DC bus, the voltage variation across the internal capacitor can be relatively large and the value of the capacitor may be made relatively small.